A receiver can mix a local reference signal with an input signal to generate an output signal at an intermediate frequency (IF), which is defined as the difference between the frequency of the input signal and the input of the local reference signal. A direct conversion receiver (DCR) synchronizes the frequency of the local reference signal with the frequency of the carrier of the input signal so that the mixer produces a zero intermediate frequency (IF) output. Thus, a direct conversion receiver may also be called a zero-IF receiver, or Synchrodyne, or Homodyne.
When a direct conversion receiver is used, the signal modulated on the desired carrier is directly demodulated. Signals modulated on other carriers can be removed with a low-pass filter that blocks the signals at the frequencies of other carriers and the frequency of the local reference signal. The receiver has high selectivity and is a precision demodulator.
A direct conversion receiver can be used to receive radio signals, such as in the applications of cell phones, televisions, avionics and medical imaging apparatus.
A direct conversion receiver may have a direct current (DC) offset problem. A reverse transmission path may occur in a direct conversion receiver. Local oscillator energy can leak through the mixer to the input and then re-enter the mixer, causing the local oscillator energy to self-mix and create a DC offset. The offset might become large enough to saturate the base band amplifiers and degrade the reception of the signals demodulated from the desired carrier.
If the direct current (DC) offset is not well suppressed, it can decrease the receiver sensitivity. A traditional solution uses a circuitry which requires a large capacitor (e.g., above 10 nH) for DC offset compensation. The size of the capacitance as required by the traditional analog circuitry is so large that such a capacitor is implemented as a discrete component.
The need of external components increases the board space and the cost of the system. Further, the external capacitor may pick up noise. For example, Ramesh Harjani, et al., describes an analog feedback circuitry to suppress the direct current (DC) offset, in “DC-Coupled IF Stage Design for a 900-MHz ISM Receiver, IEEE J Solid-State Circuits, V. 38, No. 1, pp. 126-134, 2003, which requires an external integration capacitor to detect the direct current (DC) offset.